1. Field of the Invention
The present invention relates to a Field Emission Display (FED) having an electron emitting structure which improves electron beam focusing and prevents a decrease in current density.
2. Description of the Related Art
An image display is typically used as a monitor for a Personal Computer (PC) or a television receiver. The image display can be a Cathode Ray Tube (CRT), a flat panel display such as a Liquid Crystal Display (LCD), a Plasma Display Panel (PDP), and a Field Emission Display (FED).
In the FED, electrons are emitted from an emitter regularly arranged on a cathode by supplying a strong electric field to the emitter from a gate electrode and collide with a fluorescent material coated on a surface of an anode, thereby emitting light. Since the FED forms an image by using a cool cathode electron as an electron emitting source, the image quality is highly affected by the material and structure of the emitter.
A Spindt-type metal tip (or micro tip), which is mainly composed of molybdenum, has been used as the emitter in early FEDs.
In the FED having the metal tip emitter, an ultrafine hole must be formed in order to place the emitter and molybdenum has to be deposited to form a uniform metal micro tip in the entire area of a picture plane. Thus, the manufacturing process is complicated and expensive equipment has to be used, thereby increasing the production costs of the FED. Accordingly, an FED having the metal tip emitter cannot be used for large screens.
Thus, a technique for forming a flat emitter is being studied to obtain good electron emission even with a low voltage drive and to simplify the manufacturing process.
Recently, carbon-based materials, for example, graphite, diamond, Diamond Like Carbon (DLC), C60 (Fullerene), and Carbon Nano-Tubes (CNTs) have been used for the flat emitter. Of the above materials, CNT can actively cause electron emission even at a relatively low drive voltage.
A FED having a triode structure includes a cathode, an anode, and a gate electrode. The cathode and the gate electrode are formed on a rear substrate and the anode is formed on a lower surface of a front substrate. Fluorescent layers, composed of R, G, and B phosphors, and a black matrix for improving contrast are formed on the lower surface of the anode. The rear substrate and the front substrate are spaced from each other by a spacer arranged therebetween. In such an FED, the cathode is first formed on the rear substrate, an insulating layer and the gate electrode which have fine openings are stacked thereon, and then emitters are arranged on the cathode located in the openings.
However, the FED having the triode structure as described above has low color purity during driving and has difficulty in obtaining a clear image. These problems occur because most electrons are emitted from an edge portion of the emitter and an electron beam proceeding toward the fluorescent layer diverges due to the voltage (a positive voltage of several volts through tens of volts) supplied to the gate electrode, thereby allowing a phosphor of adjacent other pixel as well as a phosphor of the intended pixel to emit light.
To resolve the above problems, an effort has been made to restrict the electron beam from the emitter from diverging by reducing the area of the emitter corresponding to one pixel to dispose a number of emitters. However, it is difficult to form a number of emitters in a pixel of a predetermined size and the entire area of the emitters for allowing a phosphor of the concerned pixel to emit light decreases. Also, the effect of focusing the electron beam is not sufficient.
In order to prevent the electron beam from diverging, a FED in which a separate electrode for focusing the electron beam is arranged around the gate electrode has been proposed.
An FED in which an electron beam is focused by disposing a ring shaped focusing electrode around the gate electrode or an FED in which an electron beam is focused by using a dual gate composed of a lower gate electrode and an upper gate electrode can be used. However, these FEDs have complicated structures. Also, since the above structures have been mainly applied to a FED having a metal tip emitter formed on the cathode, when the structures are applied to a FED having flat shape emitter, a satisfactory effect has not yet been obtained.
U.S. Pat. No. 5,552,659 relates to an electron emitting structure capable of reducing the divergence of the electron beam by defining thicknesses of a non-insulating layer and a dielectric layer which are formed on a substrate on which an emitter is arranged. However, a number of holes with respect to one pixel are formed and a fine structure composed of a number of electron emitting sources is formed in the respective hole. Thus, the structure is very complicated so that manufacturing is difficult and the structure is also spatially limited. Accordingly, there is a limitation in maximizing the number and the area of the emitter with respect to one pixel, thereby shortening the lifetime.
Also, Japanese Laid-Open Patent Publication Nos. 2000-348602, 2003-16907, and 2003-16910 relate an electron emitting structure having a flat emitter. The electron emitting structure can focus an electron beam by altering the shape of a cathode. However, the density of an electric current emitted from the emitter generally decreases, and thus, a driving voltage increases.